1. Field of the Invention
The present invention relates to the preparation and use of modified zeolite catalyst compositions which are especially suitable for the conversion of substituted aromatic hydrocarbons to provide product mixtures enriched in the para-(or 1,4-) dialkyl substituted benzene isomer.
2. Description of the Prior Art
Production of dialkyl substituted benzene compounds via disproportionation, alkylation and/or transalkylation of aromatic hydrocarbons is an important step in a number of commercial chemical manufacturing processes. Such reactions can be carried out over a variety of catalyst materials. Alkylation of aromatic hydrocarbons utilizing crystalline aluminosilicate catalysts has, for example, been described. U.S. Pat. No. 2,904,607 to Mattox refers to alkylation of aromatic hydrocarbons with an olefin in the presence of a crystalline metallic aluminosilicate having uniform openings of about 6 to 15 Angstrom units. U.S. Pat. No. 3,251,897 to Wise describes alkylation of aromatic hydrocarbons in the presence of X or Y-type crystalline aluminosilicate zeolites, specifically such type zeolites wherein the cation is rare earth and/or hydrogen. U.S. Pat. No. 4,086,287 to Kaeding et al discloses the use of ZSM-5 type zeolites as catalysts for the alkylation of aromatic hydrocarbons such as toluene. U.S. Pat. No. 3,751,504 to Keown et al. and U.S. Pat. No. 3,751,506 to Burress describe vapor phase alkylation of aromatic hydrocarbons with olefins, e.g., benzene with ethylene, in the presence of a ZSM-5 type zeolite catalyst.
The disproportionation of aromatic hydrocarbons in the presence of zeolite catalysts has been described by Grandio et al in the Oil and Gas Journal, Vol. 69, No. 48 (1971), U.S. Pat. Nos. 3,126,422; 3,413,374, 3,598,878; 3,598,879 and 3,607,961 show vapor-phase disproportionation of toluene over various catalysts.
In many of these prior art processes, the dialkylbenzene product produced contains more of the 1,3 isomer than either of the other two isomers. In the conventional methylation of toluene to form xylene, the product has the equilibrium composition of approximately 24 percent of 1,4-, 54 percent of 1,3- and 22 percent of 1,2- isomer. Of the dialkylbenzene isomers, 1,3-dialkylbenzene is often the least desired product, with 1,2- and 1,4-dialkylbenzene being the more useful products. 1,4-Dimethylbenzene, for example, is of particular value, being useful in the manufacture of terephthalic acid which is an intermediate in the manufacture of synthetic fibers such as "Dacron". Furthermore, 1,4-methylethylbenzene, i.e., para-ethyltoluene (PET), is useful for subsequent conversion to para-methylstyrene, and for this purpose ethyltoluene products containing as much as 97% of the para isomer are required.
Mixtures of dialkylbenzene isomers, either alone or in further admixture with ethylbenzene, have previously been separated by expensive superfractionation and multistage refrigeration steps. Such processes, as will be realized, involve high operation costs and have a limited yield. Alternatively, various modified zeolite catalysts have been developed to alkylate or disproportionate toluene with a greater or lesser degree of selectivity to 1,4-dialkylbenzene isomers. Hence, U.S. Pat. Nos. 3,972,832, 4,034,053, 4,128,592, and 4,137,195 disclose particular zeolite catalysts which have been treated with compounds of phosphorus and/or magnesium to increase para-selectivity of the catalysts. Para selective boron-containing zeolites are shown in U.S. Pat. No. 4,067,920 and para-selective, antimony-containing zeolites in U.S. Pat. No. 3,979,472. Similarly, U.S. Pat. Nos. 3,965,208 and 4,117,026 disclose other modified zeolites useful for shape selective reactions.
Notwithstanding the existence of such modified zeolite catalysts having para-selective properties, there is a continuing need to develop additional types of catalytic materials which are highly para-selective when used for the conversion of aromatic compounds to dialkylbenzene products. Accordingly, it is an object of the present invention to provide modified zeolite catalyst compositions which promote the conversion of aromatics to produce mixtures containing an exceptionally high percentage, e.g., 97% by weight or more, for alkylation of toluene, of para-dialkylbenzene isomer.
It is a further object of the present invention to provide such highly para-selective catalysts without necessarily resorting to expensive and/or time consuming catalyst selectivation techniques such as steaming and/or precoking after each instance of catalyst regeneration.
It is a further object of the present invention to provide highly para-selective alkylation, transalkylation and disproportionation processes employing the modified zeolite catalysts described herein.